Page:Handbook of Meteorology.djvu/267

Rh The practical unit of wind pressure is the millibar. Wind force is not always exactly proportional to the area.

The practical unit of radiation is the gram calorie (in distinction from the great calorie) or the warmth required to raise 1 gram of water 1° C in temperature.

The erg, the unit of work, is the amount of work done when a mass of 1 gram moves a distance of 1 centimeter, against a resistance of 1 dyne. A more practical unit is the joule, or 10,000,000 ergs.

The unit of time is the second, of which the mean solar day contains 86,400.

Units of Magnetism and Electricity.—Magnetism may be either positive or negative. Bodies charged with the same kind of magnetism repel each other; charged with opposite kinds, they attract. Commercial magnets are usually marked, the letter N or a dash being stamped upon the north-seeking pole. The convenient unit of magnetism is one which attracts or repels an equal quantity at a distance of 1 centimeter.

The absolute unit of current flowing through a centimeter of wire acts with a force of 1 dyne on a unit of magnetism 1 centimeter distant from every point of the wire.

An ampere, the practical unit of current, is the electromotive force of 1 volt against a resistance of 1 ohm. It is the tenth part of the absolute unit. An ordinary dry cell gives a current of about 2 amperes, a Daniels wet cell, 1 ampere. Dry cells differ somewhat in strength.

A volt, conversely, is the electromotive force, or “electric pressure,” which, flowing in a conductor having a resistance of 1 ohm, will yield 1 ampere of current. The ordinary dry cell, when fresh, has an electromotive force of about 2 volts.

The ohm is the unit of resistance. For all practical purposes it is the resistance of 50 meters of copper wire 1 millimeter in diameter. Theoretically it was intended to be the resistance of a wire in which 1 ampere of current in 1 second would generate the amount of heat equivalent to 10,000,000 ergs.

The watt is the unit of power. At the rate of 10,000,000 ergs per second, a current of 1 ampere having the pressure of 1 volt has a value of 1 watt. A common candle has the heating equivalent of about 60 watts. One horse power, the power required